1. Field of the Invention
The present invention relates to a magnetic signal reproduction system and magnetic signal reproduction method. More particularly, the present invention relates to a magnetic signal reproduction system comprising a magnetic recoding medium suited to a magnetic signal reproduction system employing a spin-valve MR head developed for high-density recording and a spin-valve MR head, and a magnetic signal reproduction method for reproducing magnetic signals in which a spin-valve MR head is employed to reproduce signals recorded on the magnetic recording medium.
2. Discussion of the Background
Magnetoresistive heads (MR heads) operating on the principle of magnetoresistive (MR) effects have been proposed in recent years. MR heads achieve several times the reproduction output of conventionally employed inductive magnetic heads. Since they do not employ induction coils, they also permit a substantial reduction in device noise such as impedance noise. Thus, MR heads are widely employed to enhance high-density recording and reproduction characteristics, particularly in hard disk drives. The mounting of MR heads in flexible disk systems has also been proposed, and MR heads are currently in use in backup tape systems.
In contrast to hard disk drives, in which signal reproduction is conducted without contact between the medium and the head, there is sliding contact between the medium and the head during reproduction in flexible disk systems and backup tape systems. Thus, when employing an MR head in such a sliding contact system, there is a problem in the form of abrasion of the head due to sliding contact with the medium. Additionally, when the smoothness of the magnetic layer surface is increased to prevent head abrasion, friction on the head increases and it becomes difficult to ensure adequate running stability.
Accordingly, controlling the number of protrusions on the surface of the magnetic layer has been proposed to achieve both low MR head abrasion and running stability. Japanese Unexamined Patent Publication (KOKAI) No. 2005-71537 or English language family member US 2005/0048324 A1 discloses such proposal. The contents of these applications are incorporated herein by reference in their entirety.
In the past, anisotropic magnetoresistive heads (AMR heads) were employed in hard disk drives. However, spin-valve MR heads, which are of even greater sensitivity, have been employed in the hard disk drives of recent years. Spin-valve MR heads have two magnetic layers. The direction of magnetization of one of the magnetic layers (called the “fixed magnetization layer” or “pinned layer”) is fixed by an antiferromagnetic layer, while the direction of magnetization of the other magnetic layer (called the “free magnetization layer” or “free layer”) changes with the external magnetic field. In spin-valve MR heads, high output can be achieved based on the difference in resistance when the directions of magnetization of these two magnetic layers are parallel and when they are antiparallel.
However, it is required to narrow the track width in spin-valve MR heads developed for high-density recording. When this is done, the MR height (the height of the spin-valve layer in the direction perpendicular to the sliding contact surface) also decreases. As a result, the amount of abrasion that is allowable decreases and performance tends to deteriorate rapidly. It is also required to reduce the length of the reproduction gap in spin-valve MR heads developed for high-density recording. When this is done, smearing (a phenomenon in which a metal material constituting the MR head is spread during sliding contact with the medium, causing electrical shorts) tends to occur.
Thus, when reproducing signals recorded on the medium described in Japanese Unexamined Patent Publication (KOKAI) No. 2005-71537 with a spin-valve MR head, for example, there are problems in that shortened head life and performance deterioration tend to occur due to head abrasion and smearing. Further, since spin-valve MR heads are highly sensitive, noise increases in spin-valve GMR heads even with protrusions of sizes that do not cause problems in AMR heads, precluding the obtaining of adequate electromagnetic characteristics. Further, the smoother the surface of the magnetic layer is made to suppress abrasion and reduce noise, the more the running stability (running property and running durability) decreases due to an increase in the coefficient of friction during running. In this manner, there is a tradeoff between smoothness of the surface of the magnetic layer (to enhance head life and reduce noise) and running stability, making it difficult to achieve both. Thus, the practical use of spin-valve MR heads has been considered difficult in sliding contact systems such as flexible disk systems and backup tape systems. However, recording of even higher density could be achieved if these problems were to be solved and spin-valve MR heads of higher sensitivity were adopted in sliding contact systems.
Further, elements tend to corrode more in spin-valve MR heads than in AMR heads. A protective layer such as a diamond-like carbon (DLC) film is provided on the outermost surface of a spin-valve MR head to increase corrosion resistance in hard disk drives. However, even when a DLC film is provided on the spin-valve MR head, the DLC film ends up being shaved away during sliding content with the medium in the above sliding contact systems. Thus, in sliding contact systems, the providing of a DLC film on a spin-valve MR head can impart resistance to corrosion while still unused, but it is difficult to maintain this corrosion resistance over an extended period during use involving sliding contact.
Accordingly, element corrosion should be solved for spin valve MR heads to be employed in sliding contact systems.